Graphite oxide

Graphite oxide, formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers. The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing.[2]

The bulk material disperses in basic solutions to yield monomolecular sheets, known as graphene oxide by analogy to graphene, the single-layer form of graphite.[3] Graphene oxide sheets have been used to prepare a strong paper-like material, and have recently attracted substantial interest as a possible intermediate for the manufacture of graphene. This goal remained elusive until 2012 since graphene obtained by this route still has many chemical and structural defects.

Graphite oxides demonstrate considerable variations of properties depending on degree of oxidation and synthesis method. For example, temperature point of explosive exfoliation is generally higher for graphite oxide prepared by Brodie method compared to Hummers graphite oxide, the difference is up to 100 degrees with the same heating rates.[6] Hydration and solvation properties of Brodie and Hummers graphite oxides are also remarkably different.[7]

Recently a mixture of H2SO4 and KMnO4 has been used to cut open carbon nanotubes lengthwise, resulting in microscopic flat ribbons of graphene, a few atoms wide, with the edges "capped" by oxygen atoms (=O) or hydroxyl groups (-OH).[8]

Graphite (Graphene) oxide (GO) has also been prepared by using a "bottom-up" synthesis method (Tang-Lau method) in which the sole source is glucose, the process is safer, simpler, and more environmentally friendly compared to traditionally “top-down” method, in which strong oxidizers are involved. Another important advantage of Tang-Lau method is control of thickness, ranging from monolayer to multilayers by adjusting growth parameters.[9]

The structure and properties of graphite oxide depend on particular synthesis method and degree of oxidation. It typically preserves the layer structure of the parent graphite, but the layers are buckled and the interlayer spacing is about two times larger (~0.7 nm) than that of graphite. Strictly speaking "oxide" is an incorrect but historically established name. Besides oxygen epoxide groups (bridging oxygen atoms), other functional groups experimentally found are: carbonyl (C=O), hydroxyl (-OH), phenol, for graphite oxides prepared using sulphuric acid (e.g. Hummers method) also some impurity of sulphur is often found, for example in a form of organosulfate groups .[10][11][12] There is evidence of "buckling" (deviation from planarity), folding and cracking [13] of graphene oxide sheets upon deposition of the layers on a choice of substrate. The detailed structure is still not understood due to the strong disorder and irregular packing of the layers.

Graphene oxide layers are about 1.1 ± 0.2 nm thick.[10][11]Scanning tunneling microscopy shows the presence of local regions where oxygen atoms are arranged in a rectangular pattern with lattice constant 0.27 nm × 0.41 nm [11][14] The edges of each layer are terminated with carboxyl and carbonyl groups.[10]X-ray photoelectron spectroscopy shows presence of several C1s peaks, their number and relative intensity depending on particular oxidation method used. Assignment of these peaks to certain carbon functionalization types is somewhat uncertain and still under debates. For example, one of interpretations goes as following: non-oxygenated ring contexts (284.8 eV), C-O (286.2 eV), C=O (287.8 eV) and O-C=O (289.0 eV).[15] Another interpretation using density functional theory calculation goes as following: C=C with defects such as functional groups and pentagons (283.6 eV), C=C (non-oxygenated ring contexts) (284.3 eV), sp3C-H in the basal plane and C=C with functional groups (285.0 eV), C=O and C=C with functional groups, C-O (286.5 eV), and O-C=O (288.3 eV).[16]

Graphite oxide is hydrophilic and easily hydrated when exposed to water vapor or immersed in liquid water, resulting in a distinct increase of the inter-planar distance (up to 1.2 nm in saturated state). Additional water is also incorporated into interlayer space due to high pressure induced effects.[17] Maximal hydration state of graphite oxide in liquid water corresponds to insertion of 2-3 water monolayers, cooling the graphite oxide/H2O samples results in "pseudo-negative thermal expansion" and below freezing point of water media results in de-insertion of one water monolayer and lattice contraction.[7] Complete removal of water from the structure seems difficult since heating at 60–80 °C results in partial decomposition and degradation of the material.

Similar to water, graphite oxide also easily incorporates other polar solvents, e.g. alcohols. However, intercalation of polar solvents occurs significantly different in Brodie and Hummers graphite oxides. Brodie graphite oxide is intercalated at ambient conditions with by one monolayer of alcohols and several other solvents (e.g. dimethylformamide and acetone) when liquid solvent is available in excess. Separation of graphite oxide layers is proportional to the size of alcohol molecule.[18] Cooling of Brodie graphite oxide immersed in excess of liquid methanol, ethanol, acetone, and dimethylformamide results in step-like insertion of additional solvent monolayer and lattice expansion. The phase transition detected by X-ray diffraction and DSC is reversible; de-insertion of solvent monolayer is observed when sample is heated back from low temperatures.[19] Additional methanol and ethanol monolayer is reversibly inserted into the structure of Brodie graphite oxide also at high pressure conditions.[18]

Hummers graphite oxide is intercalated with two methanol or ethanol monolayers already at ambient temperature. The interlayer distance of Hummers graphite oxide in excess of liquid alcohols increases gradually upon temperature decrease, reaching 19.4 and 20.6 Å at 140 K for methanol and ethanol, respectively. The gradual expansion of the Hummers graphite oxide lattice upon cooling corresponds to insertion of at least two additional solvent monolayers.[20]

Graphite oxide exfoliates and decomposes when rapidly heated at moderately high temperatures (~280–300 °C) with formation of finely dispersed amorphous carbon, somewhat similar to activated carbon.[21]

Exfoliation of graphite oxide at high temperature, screenshots from video available here:[22] Exfoliation results in tenfold increase of sample volume and formation of carbon powder with grains of few graphene layers thickness.[21]

Graphite oxide has attracted much interest as a possible route for the large-scale production and manipulation of graphene, a material with extraordinary electronic properties. Graphite oxide itself is an insulator,[23] almost a semiconductor, with differential conductivity between 1 and 5×10−3 S/cm at a bias voltage of 10 V.[23] However, being hydrophilic, graphite oxide disperses readily in water, breaking up into macroscopic flakes, mostly one layer thick. Chemical reduction of these flakes would yield a suspension of graphene flakes. It was argued that the first experimental observation of graphene was reported by Hanns-Peter Boehm in 1962.[6] In this early work the existence of monolayer reduced graphene oxide flakes was demonstrated. The contribution of Boehm was recently acknowledged by Andre Geim, the Nobel Prize winner for graphene research.[24]

Partial reduction can be achieved by treating the suspended graphene oxide with hydrazine hydrate at 100 °C for 24 hours,[15] by exposing graphene oxide to hydrogen plasma for a few seconds,[23] or by exposure to a strong pulse of light, such as that of a Xenon flash.[25] Due to the oxidation protocol, manifold defects already present in graphene oxide hamper the effectiveness of the reduction. Thus, the graphene quality obtained after reduction is limited by the precursor quality (graphene oxide) and the efficiency of the reducing agent.[26] However, the conductivity of the graphene obtained by this route is below 10 S/cm,[25] and the charge mobility is between 0.1 and 10 cm2/Vs.[23][27][28] These values are much greater than the oxide's, but still a few orders of magnitude lower than those of pristine graphene.[23] Recently, the synthetic protocol for graphite oxide was optimized and almost intact graphene oxide with a preserved carbon framework was obtained. Reduction of this almost intact graphene oxide performs much better and the mobility values of charge carriers exceeds 1000 cm2/Vs for the best quality of flakes.[29] Inspection with the atomic force microscope shows that the oxygen bonds distort the carbon layer, creating a pronounced intrinsic roughness in the oxide layers which persists after reduction. These defects also show up in Raman spectra of graphene oxide.[23]

Large amounts of graphene sheets may also be produced through thermal methods. For example, in 2006 a method was discovered that simultaneously exfoliates and reduces graphite oxide by rapid heating (>2000 °C/min) to 1050 °C. At this temperature, carbon dioxide is released as the oxygen functionalities are removed and explosively separates the sheets as it comes out.[30]

Exposing a film of graphite oxide to the laser of a LightScribe DVD has also revealed to produce quality graphene at a low cost. [31]

One suggested application of graphite oxides is water treatment.[32] Brodie graphite oxide is demonstrated to absorb selective methanol from water/methanol mixtures in certain range of methanol concentrations.[33]

Detailed studies of graphite oxide membranes (now called graphene oxide membranes or graphene oxide paper) were performed by P.-H. Boehm (German scientist who invented term "graphene") back in 1960. The paper titled "Graphite Oxide and its membrane properties" reported synthesis of "paper-like foils" with 0.05 mm thickness. The membranes were reported to be not permeable by gases (nitrogen and oxygen) but easily permeable by water vapors and, suggestively, by any other solvents which are able to intercalate graphite oxide. It was also reported that the membranes are not permeable by "substances of lower molecular weight". Permeation of water through the membrane was attributed to swelling of graphite oxide structure which enables water penetration path between individual graphene oxide layers. The interlayer distance of dried Hummers graphite oxide was reported as 6.35Å but in liquid water it increased to 11.6Å. Remarkably, the paper also cited the inter-layer distance in diluted NaOH as infinity thus reporting dispersion of graphite oxide on single-layered graphene oxide sheets in solution. The study also reported permeation rate of membranes for water 0.1 mg per minute per square cm. The diffusion rate of water was evaluated as 1 cm/hour. H.-P.Boehm's paper also shows that graphite oxide can be used as cation exchange membrane and reports measurements of osmotic pressures, membrane potentials in KCl, HCl, CaCl2, MgCl2, BaCl2 solutions. The membranes were also reported to be permeable by large alkaloid ions as they are able to penetrate between graphene oxide layers. [34] Graphene oxide membranes were also actively studied in 60-s for application in water desalination but it never come to practical applications.[35] Retention rates over 90% were reported in this study for NaCl solutions using stabilized graphene oxide membranes in reverse osmosis setup.

In 2013 Lockheed Martin announced their Perforene graphene filter. Lockheed claims the filter reduces energy costs of reverse osmosis desalination by 99%. Lockheed claimed that the filter is 500 times thinner than the best filter on the market today and a thousand times stronger and requires 1% of the pressure.[36]

Graphene oxide membranes were suggested recently for applications in filtering water. The announced specimens allow ions from common salts to pass the filter but retain some larger size ions.[36] Narrow capillaries allow rapid permeation, by mono- or bilayer water. Multilayer stacks (laminates) have a structure similar to nacre, which provides mechanical strength at water free conditions. Helium cannot pass through the membranes in humidity free conditions but penetrates easily exposed to humid gas, whereas water vapor passes with no resistance. Dry laminates are vacuum-tight, but immersed in water, act as molecular sieves, blocking some solutes with hydrated radii larger than 4.5 angstroms.[37]

A second project produced graphene sheets with subnanoscale (0.40 ± 0.24 nm) pores. The graphene was bombarded with gallium ions, which disrupt carbon bonds. The result is etched with an oxidizing solution produces a hole at each spot struck by a gallium ion. The length of time the graphene sheet is in the oxidizing solution determines average pore size. Pore density, reached 5 trillion pores per square centimeter, while retaining structural integrity. The pores permitted cations transport at short oxidation times, consistent with electrostatic repulsion from negatively charged functional groups at pore edges. At longer oxidation times, sheets were permeable to salt but not larger organic molecules.[38]

Optically transparent, multilayer films made from graphene oxide are impermeable under dry conditions. Exposed to water (or water vapor), they allow passage of molecules below a certain size. The films consist of millions of randomly stacked flakes, leaving nano-sized capillaries between them. Closing these nanocapillaries using chemical reduction with hydroiodic acid creates “reduced graphene oxide” (r-GO) films that are completely impermeable to gases, liquids or strong chemicals greater than 100 nanometers thick. Glassware or copper plates covered with such a graphene "paint” can be used as containers for corrosive acids. Graphene-coated plastic films could be used in medical packaging to improve shelf life.[39]

Graphene oxide has been used in DNA analysis applications. The large planar surface of graphene oxide allows simultaneous quenching of multiple DNA probes labeled with different dyes, providing the detection of multiple DNA targets in the same solution. Further advances in graphene oxide based DNA sensors could result in very inexpensive rapid DNA analysis.[40] Recently a group of researchers, from university of L'Aquila (Italy), discovered new wetting properties of grafene oxide thermally reduced in ultra-high vacuum up to 900 °C. They found a correlation between the surface chemical composition, the surface free energy and its polar and dispersive components, giving a rationale to the wetting properties of graphene oxide and reduced graphene oxide.[41][42]

Graphene oxide has been demonstrated as a flexible free-standing battery anode material for room temperature lithium-ion [43] and sodium-ion batteries.[44][45] It is also being studied as a high surface area conducting agent in lithium-sulfur battery cathodes.[46]